Cathode electroactive material, production method therefor and secondary cell

ABSTRACT

A cathode electroactive material for use in lithium ion secondary cells, process for producing the material, and lithium ion secondary cells using the cathode electroactive material, wherein the electroactive material predominantly comprises an Li—Mn composite oxide particles with the spinel structure and particles of the electroactive material have an average porosity of 15% or less, the porosity being calculated by employing the following equation:
 
Porosity (%)=( A/B )×100 
 
(wherein A represents a total cross-section area of pores included in a cross-section of one secondary particle, and B represents the cross-section area of one secondary particle), a tapping density of 1.9 g/ml or more, a size of crystallites of 400 Å-960 Å, a lattice constant of 8.240 Å or less. The cathode electroactive material of the present invention is formed of particles which are dense and spherical and exhibit excellent packing characteristics to an electrode, and exhibit high initial capacity and capacity retention percentage at high temperature.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 09/785,258, filed Feb. 20,2001, now U.S. Pat. No. 6,699,618 the disclosure of which isincorporated herein by reference, which claims benefit pursuant to 35U.S.C. §119(e)(1) of the filing date of Provisional Application60/214,794 filed under Jun. 28, 2000 pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to a cathode electroactive material foruse in lithium ion secondary cells, a process for producing thematerial, and a lithium ion secondary cell using the cathodeelectroactive material.

BACKGROUND ART

Lithium manganese composite oxides (hereinafter referred to as Li—Mncomposite oxides), which are very safe and are produced from abundantnatural resources, have been of interest for use as a cathodeelectroactive material for lithium ion secondary cells. However, Li—Mncomposite oxides exhibit poor discharge capacity per amount of anelectroactive material as compared with lithium cobalt composite oxides(hereinafter referred to as Li—Co composite oxides). In addition,secondary particles of Li—Mn composite oxide are lightweight and absorba large amount of oil, because the particles contain many pores. Thus,the amount of electroactive material which can be fed into adimensionally limited cell must be restricted, thereby disadvantageouslylowering the electrochemical capacity of a unit cell.

In recent years, U.S. Pat. No. 5,807,646 (Japanese Patent ApplicationLaid-Open (kokai) No. 9-86933) has proposed measures to counter theaforementioned problem. Specifically, a mixture of a manganese compoundand a lithium compound is shaped at a pressure of 500 kg/cm² or higher,heated, and crushed, to thereby produce an Li—Mn composite oxide havinga tapping density (i.e., apparent density of powder in a container whichis moved, e.g., vibrated under certain conditions) of 1.7 g/ml orhigher. However, the disclosed tapping density is at most 1.9 g/ml,which is unsatisfactory.

The above official gazette also discloses the average particle size ofsecondary particles which are formed by aggregating primary particles ofan Li—Mn composite oxide. However, even when the packing density ofsecondary particles is enhanced through the interaction between primaryparticles, secondary particles are disintegrated during the electrodematerial (paste) preparation step. Thus, controlling the averageparticle size of the secondary particles is not a fundamentalcounter-measure.

Some methods for producing a spinel-type Li—Mn composite oxide havealready been disclosed. Japanese Patent Application Laid-Open (kokai)No. 9-86933 discloses such a method comprising burning a mixture of amanganese compound and a lithium compound at a high temperature, e.g.,250° C. to 850° C. Japanese Patent Application Laid-Open (kokai) No.4-237970 discloses such a method comprising mixing a manganese compound,a lithium compound, and an oxide of boron which can be substituted bymanganese and burning the resultant mixture at a high temperature, tothereby produce an Li—Mn—B oxide in which Mn atoms are partiallysubstituted with B, and the Li—Mn—B oxide serves as a cathodeelectroactive material.

When the aforementioned materials are burned at high temperature in airor in an oxygen gas flow, the secondary particles obtained throughcrushing have a high average porosity (15% or more) and a low tappingdensity (1.9 g/ml or less). Thus, thus-obtained cathode electroactivematerials cannot be charged into an electrode in a large amount, andthereby, a high discharge capacity cannot be attained.

Japanese Patent Application Laid-Open (kokai) No. 4-14752 discloses acathode electroactive material employing a manganese oxide which isproduced by mixing spinel-type lithium manganese oxide and titaniumoxide and sintering the resultant mixture. However, disadvantageously,titanium oxide only reacts with lithium and manganese at 950° C. to1000° C. or higher to form a melt, and a tapping density of 1.60 g/mlcan be only attained by adding titanium oxide in an amount as large as10 mass %.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a cathode electroactivematerial for us in lithium ion secondary cells, which electroactivematerial has an excellent packing property and exhibits a high initialdischarge capacity and a low decrease in discharge capacity aftercharging and discharging are repeated (hereinafter the property isreferred to as high “capacity retention”).

The present inventors have conducted extensive studies, and have solvedthe aforementioned problems by successfully densifying particles of anLi—Mn composite oxide. Specifically, the spinel-type Li—Mn compositeoxide is burned and crushed. Then, a sintering agent is added to theresultant pulverized particles, and the particles are granulated andburned.

Accordingly, the present invention provides a cathode electroactivematerial for use in lithium ion secondary cells, a process for producingthe material, a paste for producing an electrode and a cathode electrodefor use in lithium ion secondary cells comprising a cathodeelectroactive material, and a lithium lon secondary cell as describedbelow.

[1] A cathode electroactive material for use in lithium ion secondarycells, wherein the cathode electroactive material predominantlycomprises Li—Mn composite oxide particles with the spinel structure andparticles of the electroactive material have an average porosity of 15%or less, the porosity being expressed by the following equation:Porosity (%)=(A/B)×100 . . .  (1)(wherein A represents a total cross-section area of pores included in across-section of one secondary particle, and B represents thecross-section area of one secondary particle).

[2] A cathode electroactive material for use in lithium ion secondarycells as described in [1], wherein the average porosity is 10% or lessand the average particle size of primary particles is 0.2 μm-3 μm.

[3] A cathode electroactive material for use in lithium ion secondarycells as described in [1], wherein the tapping density of the cathodeelectroactive material is 1.9 g/ml or more.

[4] A cathode electroactive material for use in lithium ion secondarycells as described in [3], wherein the tapping density of the cathodeelectroactive material is 2.2 g/ml or more.

[5] A cathode electroactive material for use in lithium ion secondarycells as described in [1], wherein the size of crystallites contained inthe cathode electroactive material is 400 Å-960 Å.

[6] A cathode electroactive material for use in lithium ion secondarycells as described in [1], wherein the lattice constant determined withrespect to the electroactive material is 8.240 Å or less.

[7] A cathode electroactive material for use in lithium ion secondarycells as described in [1], wherein the electroactive material isproduced by granulating an Li—Mn composite oxide with the spinelstructure serving as a predominant component comprising an oxide whichis molten at 550° C.-900° C.: an element which forms the oxide: acompound comprising the element; an oxide which forms a solid solutionor melts to react with lithium or manganese: an element which forms theoxide: or a compound comprising the element, and sintering the formedgranules.

[8] A cathode electroactive material for use in lithium ion secondarycells as described in [7], wherein the oxide which is molten at 550°C.-900° C.: or the element which forms the oxide: or the compoundcomprising the element; or the oxide which forms a solid solution ormelts to react with lithium or manganese: or the element which forms theoxide: the compound comprising the element, is at least one elementselected from the group consisting of Bi, B, W, Mo, and Pb: or acompound comprising the element; a compound comprising B₂O₃ and LiF; ora compound comprising MnF₂ and LiF.

[9] A process for producing a cathode electroactive material for use inlithium ion secondary cells predominantly comprising an Li—Mn compositeoxide with the spinel structure, which comprises adding, to a pulverizedLi—Mn composite oxide with the spinel structure, an oxide which ismolten at 550° C.-900° C.: an element which forms the oxide: a compoundcomprising the elements an oxide which forms a solid solution or meltsto react with lithium or manganese: an element which forms the oxide: ora compound comprising the element; and mixing, to thereby form granules.

[10] A process for producing a cathode electroactive material for use inlithium ion secondary cells as described in [9], which process comprisessintering the granules in addition to forming granules.

[11] A process for producing a cathode electroactive material for usein, lithium ion secondary cells as described in [9], which processcomprises, in addition to forming granules, sintering the granules byelevating the temperature of the granules from asintering-shrinkage-initiating temperature to a temperature higher thanthe sintering-shrinkage-initiating temperature by at least 100° C. at arate of at least 100° C./minute; successively maintaining the elevatedtemperature for one minute-10 minutes; and lowering the temperature to asintering-initiating temperature at a rate of at least 100° C./minute.

[12] A process for producing a cathode electroactive material for use inlithium ion secondary cells as described in [11], wherein the sinteringis carried out by use of a rotary kiln.

[13] A process for producing a cathode electroactive material for use inlithium ion secondary cells as described in [10], wherein at least oneelement selected from the group comprising of Bi, B, W, Mo, and Pb: thecompound comprising the element; a compound comprising B₂O, and LiF; ora compound comprising MnF₂ and LiF is molten on the surfaces ofparticles of Li—Mn composite oxide so as to carry out the abovedescribed sintering process.

[14] A process for producing a cathode electroactive material for use inlithium ion secondary cells as described in [9], wherein pulverizedLi—Mn composite oxide with the spinel structure has an average particlesize of 5 μm or less.

[15] A process for producing a cathode electroactive material for use inlithium ion secondary cells as described in [9], wherein pulverizedLi—Mn composite oxide with the spinel structure has an average particlesize of 3 μm or less.

[15] A process for producing a cathode electroactive material for use inlithium ion secondary cells as described in [9], wherein granulationprocess is carried out through spray granulation, agitation granulation,compressive granulation, or fluidization granulation.

[17] A process for producing a cathode electroactive material for use inlithium ion secondary cells as described in [9], wherein at least oneorganic compound selected from the group consisting of acrylic resin, anisobutylene-maleic anhydride copolymer, poly(vinyl alcohol),poly(ethylene glycol), polyvinylpyrrolidene, hydroxypropyl cellulose,methyl cellulose, cornstarch, gelatin, and lignin is employed as agranulation aid during granulation process.

[18] A process for producing a cathode electroactive material for use inlithium ion secondary cells as described in [17], which processcomprises binder removal process in air or in an oxygen-containingenvironment at 300° C. to 550° C.

[19] A cathode electroactive material for use in lithium ion secondarycells which is produced through a process as described in any one of [9]to [18].

[20] A paste for producing an electrode comprising a cathodeelectroactive material for use in lithium ion secondary cells as claimedin any one of claims [1] to [8].

[21] A cathode electrode for a lithium lon secondary cell, which theelectrode comprises a cathode electroactive material for use in lithiumion secondary cells as described in any of [1] to [8] or [19].

[22] A lithium ion secondary cell equipped with a cathode electrode fora lithium ion secondary cell as described in [21].

[23] A lithium ion secondary cell as described in [22], which is formedinto a coin-shaped cell, a coil cell, a cylinder-shaped cell, abox-shaped cell, or a lamination cell.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example (Example 14) of photographic images (scanningelectron microscope, ×15,000) of the cathode electroactive materialaccording to the present invention, which was granulated, burned, andsize-adjusted.

FIG. 2 shows an example (Example 14) of particle size distribution ofthe cathode electroactive material according to the present invention,which was granulated, burned, and size-adjusted.

DETAILED DESCRIPTION OF INVENTION

The present invention will next be described in detail.

The present invention relates to a spinel-type Li—Mn composite oxide, inwhich secondary particles of the electroactive material have a porosityof 15% or less. The porosity is considerably reduced as compared withthe electroactive material of a conventional electrode. The presentinvention also relates to a spinel-type Li—Mn composite oxide, in whichsecondary particles of the oxide have an average porosity of 10% orless. The electrochemical cycle characteristics of the oxide are moreexcellent than those of a conventional Li—Mn composite oxide.

The cathode electroactive material of the present invention comprising aspinel-type lithium-magnesium (Li—Mn) composite oxide collectivelyrefers to compounds represented by LiMn₂O₄, Li_(1−x)Mn_(2−x)O₄(0<x<0.2),or Li_(1−x)Mn_(2−x−y)M_(y)O₄(0<x<0.2, 0<y<0.4) in which Mn is partiallysubstituted by at least one element (represented by M in the formula)selected from the group consisting of chromium, cobalt, aluminum,nickel, iron, and magnesium.

The cathode electroactive material of the present invention for use inlithium ion secondary cells, in which the electroactive materialpredominantly comprises a spinel-type LL-Mn composite oxide andsecondary particles of the electroactive material have an averageporosity of 15% or less, the porosity of one secondary particle beingcalculated by employing the following equation:Porosity (%)=(A/B)×100 . . .  (1)(wherein A represents a total cross-section area of pores included in across-section of one secondary particle, and B represents thecross-section area of one secondary particle).

In aforementioned Li—Mn composite oxide, the average porosity of theaforementioned cathode electroactive material is preferably 10% or less,and the average particle size of primary particles is 0.2 μm-3 μm.

In order to attain a tapping density of the cathode electroactivematerial in excess of 1.9 g/ml, the average porosity of the secondaryparticles is required to be 15% or less, preferably 13% or less, morepreferably 10% or less. When sintering is carried out at a hightemperature for a long period of time in a typical process for producinga composite oxide so as to reduce the average porosity of secondaryparticles to be as low as possible through sintering, primary particlesare grown to large particles as sintering proceeds. Employment of thethus-produced material as a cathode electroactive material for a cellresults in a decrease in the capacity retention of the cell. Thus, cellsfabricated from the material have poor cell performance.

The present inventors have conducted extensive studies on a method forsintering with suppressing particle growth, and have found thatsintering with suppressing particle growth can be brought about byelevating the temperature of granules from asintering-shrinkage-initiating temperature to a temperature higher thanthe sintering-shrinkage-initiating temperature (as measured throughthermo-mechanical analysis) by at least 100° C. at a rate of at least100° C./minute; successively maintaining the elevated temperature forone minute to 10 minutes; and lowering the temperature to asintering-initiating temperature at a rate of at least 100° C./minute.

The term “sintering-shrinkage-initiating temperature” herein refers to ashrinkage-initiating temperature measured through thermo-mechanicalanalysis. The aforementioned maintained temperature is required to behigher than the sintering-shrinkage-initiating temperature by at least100° C. When the maintained temperature is elevated by less than 100° C.the sintering-shrinkage rate is small, leading to a longer sinteringtime. As a result, particles are grown to a primary particles size ofmore than 0.5 μm. The time for maintaining the elevated temperature isone minute or longer and 10 minutes or shorter so as to attain a primaryparticle size of 0.2 μm or more and 0.5 μm or less and excellent cellcharacteristics. The temperature is higher than thesintering-shrinkage-initiating temperature by at least 100° C. duringthe aforementioned sintering step. To maintain the temperature for lessthan one minute is not sufficient for thermal conduction, and particleshaving a primary particle size as small as less than 0.2 μm and poorcrystallinity will be produced, thereby lowering the initial dischargecapacity. When the time is in excess of 10 minutes, particles continueto grow after sintering is completed, thereby elevating the primaryparticle size and lowering the capacity retention percentage.Accordingly, in the present invention, the time for maintaining thetemperature is preferably 2-8 minutes, more preferably 2-5 minutes.

Temperature elevating and lowering rates between thesintering-initiating temperature and the temperature for maintaining theelevated temperature are set to be at least 100° C./minute for thefollowing reasons: The time during which the temperature is maintainedin the temperature region where particles are grown is made as short aspossible, for allowing only sintering to proceed, preventing growth ofparticles.

In order to attain a tapping density of the cathode electroactivematerial in excess of 2.2 g/ml, the average porosity of the secondaryparticles is required to be 10% or less, preferably 7% or less, morepreferably 5% or less.

In the present invention, the size of crystallites comprised in theaforementioned cathode electroactive material is preferably 400 Å-960 Å.When the size is less than 400 Å, crystallinity is insufficient, therebylowering the initial discharge capacity of the cell and the capacityretention percentage, whereas when the size is in excess of 960 Å, thecapacity retention percentage drastically decreases. More specifically,the size is preferably 500 Å-920 Å, more preferably 700 Å-920 Å.

The lattice constant determined with respect to the cathodeelectroactive material of the present invention comprising a spinel-typeLi—Mn composite oxide is preferably 8.240 Å or less. When the latticeconstant is in excess of 8.240 Å, the capacity retention percentagedrastically decreases. Accordingly, the lattice constant is preferably8.235 Å or less, more preferably 8.233 Å or less.

The cathode electroactive material of the present inventionpredominantly comprising a spinel-type Li—Mn composite oxide is formedof dense granulated particles which are prepared by crushing a burnedspinel-type Li—Mn composite oxide; adding a sintering agent (granulationaccelerator) to the resultant pulverized particles (i.e., secondaryparticles which are formed by aggregating primary particles andpreferably have an average particle size of 0.5 μm or less); and burningto granulate. The term “dense granulated particles” herein refers toparticles in which no or few pores are contained between primaryparticles of the oxide. The cathode electroactive material of thepresent invention is formed of the aforementioned dense substance, andis formed by employment of a sintering agent mentioned in below.

Hereinafter the process for producing the cathode electroactive materialof the present invention will be described.

The process for producing a spinel-type Li—Mn composite oxide comprisesburning a mixture containing a manganese compound, a lithium compound,and an optional compound containing a hetero-element which can besubstituted by manganese, in air or an oxygen gas flow at 300° C.-850°C. for at least one hour.

No particular limitation is imposed on the crystallinity of thespinel-type Li—Mn composite oxide, and an unreacted lithium compound ormanganese compound may remain in the composite oxide. When thespinel-type Li—Mn composite oxide has a high crystallinity, the latticeconstant thereof is not particularly limited. However, employment of aspinel-type Li—Mn composite oxide having a lattice constant of 8.240 Åor less as a cathode electroactive material prevents decrease incapacity retention percentage.

No particularly limitation is imposed on the raw material for producingthe spinel-type Li—Mn composite oxide, and known manganese compoundssuch as manganese dioxide, dimanganese trioxide, trimanganesetetraoxide, hydrated manganese oxide, manganese carbonate, and manganesenitrate; and lithium compounds such as lithium hydroxide, lithiumcarbonate, and lithium nitrate are employed.

Preferably, manganese carbonate is suitable for the aforementionedmanganese compound in that manganese carbonate readily reacts with alithium compound at low temperature. An Li—Mn oxide of cathodeelectroactive material obtained from manganese carbonate impartsexcellent properties to cells. In order to produce manganese-substitutedLi—Mn—M (hetero-element) composite oxide represented byLi_(1+x)Mn_(2−x−y)M_(y)O₄, at least one element selected from the groupconsisting of chromium, cobalt, aluminum, nickel, Iron, and magneseiumis added to the aforementioned manganese compound and lithium compoundserving as the raw materials. Any M-containing compound (hetero-element)can be used so long as the compound forms the aforementioned oxidethrough thermal reaction, and the M-containing compound may be added tothe manganese compound and lithium compound during thermal reaction.

No particular limitation is imposed on the method for crushing andpulverizing secondary particles of the aforementioned spinel-type Li—Mncomposite oxide, and known crushers and pulverizers can be employed.Examples include a medium-stirring type pulverizer, a ball mill, a paintshaker, a jet-mill, and a roller mill. Crushing and pulverizing may beperformed in a dry manner or a wet manner. No particular limitation isimposed on the solvent employed in the wet-manner crushing andpulverizing, and solvents such as water and alcohol may be employed.

Particle size of the crushed and pulverized spinel-type Li—Kn compositeoxide is important in view of acceleration of sintering. The particlesize measured by means of a laser particle size distribution measurementapparatus is preferably 5 μm or less. More preferably, no coarseparticles having a size 5 μm or more is contained, and containedparticles have an average particle size of 2 μm or less. Still morepreferably, no coarse particles having a size more than 3 μm iscontained, and contained particles have an average particle size of 1.5μm or less. The particle size is further preferably 0.5 μm or less, yetfurther preferably 0.3 μm or less, particularly preferably 0.2 μm orless.

No particular limitation is imposed on the method for mixing a sinteringagent with the crushed and pulverized spinel-type Li—Mn composite oxide.For example, mixing may be carried out by use of a medium-stirring typecrushing machine, a ball mill, a paint shaker, or a mixer. Mixing may beperformed in a dry manner or a wet manner. The sintering agent may beadded to the Li—Mn composite during crushing and pulverizing the oxide.

The sintering agent is not particularly limited, so long as it enablessintering of crushed and pulverized particles of the Li—Mn compositeoxide for granulation of the particles. The sintering agent ispreferably a compound which melts at 900° C. or lower. For example, thecompound may be an oxide which melts at 550-900° C. or a precursor whichmay be converted into the oxide; or an oxide which forms a solidsolution with lithium or manganese or reacts with lithium or manganeseto form a melt, or a compound which may be converted into the oxide.

The sintering agent, for example, may be a compound comprising anelement such as Bi, B, W, Mo, or Pb. Such compounds may be employed incombination. The sintering agent may be a compound comprising B₂O₃ andLiF, or a compound comprising MnF₂ and LiF. The sintering agent is morepreferably a compound comprising Bi, B, or W, since such a compoundgreatly exerts sintering (sintering-shrinkage) effect.

Examples of Bi compounds include bismuth trioxide, bismuth nitrate,bismuth benzoate, bismuth hydroxyacetate, bismuth oxycarbonate, bismuthcitrate, and bismuth hydroxide. Examples of B compounds include boronsesquioxide, boron carbide, boron nitride, and boric acid. Examples of Wcompounds include tungsten dioxide and tungsten trioxide. The amount ofa sintering agent which is added to the composite oxide is 0.0001-0.05mol (as reduced to metallic element in the agent) on the basis of 1 molof Mn in the Li—Mn composite oxide. When the amount is less than 0.0001mol, the sintering agent exerts no sintering (sintering-shrinkage)effect, whereas when the amount is in excess of 0.05 mol, the initialcapacity of the electroactive material comprising the composite oxidebecomes low. The amount to be added is preferably 0.005-0.03 mol.

The sintering agent may be used in the form of powder, or may bedissolved in a solvent and used in the form of solution. When thesintering agent is employed in the form of powder, the agent preferablyhas an average particle size of 50 μm or less, more preferably 10 μm orless, much more preferably 3 μm or less. The sintering agent ispreferably added to the crushed composite oxide particles beforegranulation and sintering of the particles. Alternatively, aftergranulation of the particles, resultant granules may be impregnated withthe sintering agent at a temperature at which the agent melts, and thensintering may be carried out.

A sintering agent often remains after the sintering step in the cathodematerial for use in cells. For example, the aforementioned sinteringagent used in the producing process of the present invention is detectedby analysis to remain in the cathode electroactive material.

A method for granulation will next be described.

Granulation may be carried out by use of the aforementioned sinteringagent through spray granulation, flow granulation, compressiongranulation, or stirring granulation. The granulation may be carried outin combination with medium-flow drying or medium-vibration drying.

In the present invention, no particular limitation is imposed on themethod for granulation so long as dense secondary particles (includinggranulated particles) are formed. Stirring granulation and compressiongranulation are particularly preferred in consideration of production ofsecondary particles having a high density. Spray granulation is alsoparticularly preferred in consideration of production of granules havinga round shape. Examples of stirring granulation apparatuses include avertical granulator (product of Paurec) and Spartanryuzer (product ofFuji Paudal). Examples of compression granulation apparatuses include aroller compactor (model: MRCP-200, product of Kurimoto Tekko). Examplesof spray granulation apparatuses include a mobile-minor-type spray dryer(product of Ashizawaniro Atomizer).

No particular limitation is imposed on the size of secondary particlesto be granulated. When the average size of the granulated secondaryparticles is very large, the particles may be lightly crushed andpulverized immediately after granulation or after sintering of theparticles, and then subjected to size-regulation such as classification,to thereby obtain the granules of desired size. Typically, secondaryparticles having an average particle size of 10-20 μm are preferred.

In order to enhance granulation efficiency, an organic granulation aidmaybe added.

Examples of these granulation aids include an acrylic resin, anisobutylene-maleic anhydride copolymer, poly(vinyl alcohol),poly(ethylene glycol), polyvinylpyrrolidone, hydroxypropyl cellulose,methyl cellulose, cornstarch, gelatin, and lignin.

Although the granulation aid may be added in the form of powder, thegranulation aid is preferably added by spraying it dissolved in water oran organic solvent such as alcohol in view of granulation efficiency.The granulation aid is added preferably in an amount of five parts byweight or less on the basis of 100 parts by weight of a mixture of thesintering agent and the spinel-type Li—Mn composite oxide, morepreferably two parts by weight or less.

A method for sintering the granulated secondary particles will next bedescribed.

The binder contained in the granulated secondary particles is removed at300-550° C. for 10 minutes or more in air or in an oxygen-containing gasflow. The amount of residual carbon in the binder-free granules ispreferably 0.1% or less.

In order to proceed sintering with suppressing growth of particles,binder-removed granules are fired in air or an oxygen-containing gasflow at 550° C. to 900° C. for one minute or longer. Under theseconditions, the sintering agent is maintained molten on Li—Mn compositeoxide particles, thereby densifying secondary particles throughsintering.

In the present invention, binder-free granulated particles is burned inair or an oxygen-containing gas flow under the following conditions soas to suppress growth of particles and proceed sintering. Specifically,the procedure includes elevating the temperature from asintering-shrinkage-initiating temperature measured throughthermo-mechanical analysis to a temperature higher than thesintering-shrinkage-initiating temperature by at least 100° C. at a rateof at least 100° C./minute; successively maintaining the elevatedtemperature for one minute-10 minutes; and lowering the temperature to asintering-initiating temperature at a rate of at least 100° C./minute,to thereby attain sintering and densify the secondary particles.Temperature elevation and lowering between ambient temperature and thesintering-shrinkage-initiating temperature may be 10° C./min or less ashas been conventionally employed.

Even when the aforementioned organic granulation aid is not employed,sintering of the granules may be carried out in air or in anoxygen-gas-flow atmosphere in a manner as described above, to therebyproduce dense secondary particles.

The cathode electroactive material of the present invention and thecathode electroactive material produced through the method for producingthe same according to the present invention are formed into a cathodeelectrode of lithium ion secondary cells, and performance of the cellare evaluated through methods similar to those employed for aconventional Li—Mn composite oxide.

Hereinafter, example methods for employing the cathode electroactivematerial of the present invention as a material of a cathode electrodein the non-aqueous secondary cell will next be described.

The cathode material is produced through the following procedure:kneading the cathode electroactive material, a conductivity-impartingagent such as carbon black or graphite, and a binder such aspolyvinylidenefluoride dissolved in a solvent (e.g.,N-methylpyrrolidone) in predetermined proportions; applying theresultant electrode paste to a current-collecting material; drying; andpressing the paste-applied material by use of a roll press or a similarapparatus. The current-collecting material may be a known metalliccurrent-collecting material such as aluminum, stainless steel, ortitanium.

In the non-aqueous secondary cell according to the present invention, anelectrolytic salt contained in an electrolytic solution may be a knownfluorine-containing lithium salt. For example, LiPF₆, LiBF₄,LIN(CF₃SO₂)₂, LiAsF₆, LiCF₃SO₃, or LiC₄F₉SO₃ may be employed. Theelectrolytic solution employed in the non-aqueous secondary cell isproduced by dissolving at least one species of the aforementioned knownfluorine-containing lithium salts in a non-aqueous electrolyticsolution. The aforementioned non-aqueous solvent for the non-aqueouselectrolytic solution is not particularly limited, so long as thesolvent is chemically or electrochemically stable and aprotic.

Examples of such solvents include carbonic acid esters such as dimethylcarbonate, propylene carbonate, ethylene carbonate, methyl ethylcarbonate, methyl propyl carbonate, methyl isopropyl carbonate, methylbutyl carbonate, diethyl carbonate, ethyl propyl carbonate, diisopropylcarbonate, dibutyl carbonate, 1.2-butylene carbonate, ethyl isopropylcarbonate, and ethyl butyl carbonate; oligoethers such as triethyleneglycol methyl ether and tetraethylene glycol dimethyl ether; aliphaticesters such as methyl propionate and methyl formate; aromatic nitrilessuch as benzonitrile and tolunitrile; amides such as dimethylformamide;sulfoxides such as dimethyl sulfoxide; lactones such as y-butyrolactone;sulfur compounds such as sulforane; N-vinylpyrrolidone;N-methylpyrrolidone; and phosphoric acid esters. Of these, carbonic acidesters, aliphatic esters, or ethers are preferred.

In the non-aqueous secondary cell of the present invention, the materialof an anode electrode is not particularly limited, so long as it canreversibly occlude or release lithium ions. For example, the materialmay be lithium metal, lithium alloy, carbon material (includinggraphite), or metal-chalcogen.

A method for evaluation of electrode characteristics will next bedescribed.

The cathode electroactive material, Vulcan XC-72 (product of CabotCorp.) serving as a conductive material, and an ethylenetetrafluorideresin serving as a binder are mixed in proportions by weight of50:34:16, and the resultant mixture is swollen with toluene over 12hours. The swollen mixture is applied onto a current-collecting materialcomprising aluminum expanded metal, and shaped at a pressure of 2 t/cm²,and then toluene is dried, to thereby produce a cathode electrode. Ananode electrode is produced from lithium foil.

Propylene carbonate and dimethyl carbonate are mixed at a ratio byvolume of 1:2, and LiPF₄ is dissolved in the resultant mixture in aconcentration of 1 mol/liter, to thereby produce an electrolyticsolution. A separator formed of polypropylene is employed. In order toprevent micro short circuit due to formation of dendrite in the anodeelectrode, for example, silica fibrous filter paper QR-100 (product ofAdvantec Toyo Co.) serving as a reinforcing material is employed incombination. A 2016-type coin-shaped cell is fabricated from the cathodeelectrode, the anode electrode, the electrolytic solution, theseparator, and the reinforcing material. The thus-fabricated cell issubjected to charging and discharging test of 500 cycles in a thermostatof 60° C. Measurement conditions are as follows:constant-current-constant-voltage charging and constant-currentdischarging; charging or discharging rate 1C (charging time: 2.5 hours);and scanning voltage 3.1-4.3 V.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described below by referring to Examples andComparative Examples, however, the present invention should not beconstrued as being limited thereto.

The characteristics of the cathode electroactive material as shown inexamples below and Tables 1-3 was evaluated according to the followingprocedures.

1) Average Particle Size and Specific Surface Area

The powder was dispersed in a 0.2% aqueous solution of Demol P (KaoCorporation) by the application of ultrasound, and the particle sizedistribution was measured by means of a laser particle size distributionmeasuring apparatus (GRANULOMETER, Model HR 850, product of CILAS).

2) Tapping Density

The tapping density was measured by vertically vibrating 2000 times atan amplitude of 8 mm using a tapping machine (type KRS-409, KuramochiKagaku Kiki Seisakusho).

3) Porosity

The cathode electroactive material was embedded in resin by mixing andhardening the cathode electroactive material and thermosetting resin,and the resin solid was cut by means of a microtome. The cut, surfacewas mirror-polished, and the thus-polished surface was observed byScanning Electron Microscope (SEM). The cross-section area of onesecondary particle (B) calculated from the obtained SEM photographicimage and the total cross-section area of all pores (A) included in thecross-section area of one secondary particle were determined by means ofan image-analyzer. The porosity (C) (%) of one secondary particle wascalculated according to the following formula to determine the averageporosity from the average of 50 secondary particles selected at random:C(%)=(A/B)×100.4) Crystallite Size

The crystallite size was determined by employing Sherrer's formula fromthe peak corresponding to a (111) face as measured under the followingconditions through X-ray diffractometry.

On the assumption that the crystallites are cubic and constant in size,broadening of the diffraction peak depending on the size of crystallitewas calculated on the basis of the half-width. Monocrystalline siliconewas pulverized using a sample mill made of tungsten carbide and sievedto a size of 44 μm or less. The apparatus constant calibration curve wasdetermined by the employment the sieved powder as an external standard.

[Measurement Apparatus and Method]

The measurement apparatus employed to analyze the size of thecrystallites was a Rad-type goniometer (Rigaku Denki) (measurement mode:continuous), and the analysis software employed was RINT 2000 Series(application software, Rigaku Denki).

The measurement conditions were as follows: X-ray; CuKα ray, outputpower; 50 kV. 180 mA: slit widths (3 points): ½°, ½°, and 0.15 mm,scanning method; 2θ/θ, scanning rate: 1°/min; measuring range (2θ);17-20°, and step: 0.004°. The measurement accuracy for the crystallitesize fell within ±30 Å.

5) Lattice Constant

The lattice constant was obtained through a method described by J. B.Nelson and D. P. Riley (Proc. Phys. Soc., 57, 160 (1945)).

6) Specific Surface Area

Specific Surface Area was obtained according to BET method.

7) Shape of Granulated Particles

Granules of cathode electroactive materials were photographed by SEM.Through analysis of the obtained images, roundness(roundness=4π[area/(circumference)²]) and aspect ratio (aspectratio=absolute maximum length of needle/diagonal width) of secondaryparticles were obtained. The average values of 200 secondary particleswere measured for each sample.

EXAMPLE 1

Manganese carbonate having a specific surface area of 22 m²/g (C2-10;product of Chuo Denki Kogyo) and lithium carbonate (3N, product of HonjoChemical) were mixed together at an element ratio of 0.51 (Li/Mn) usinga ball mill. The resultant mixture was heated from room temperature to650° C. at a rate of 200° C./hour in air. The temperature was maintainedfor four hours, to thereby obtain an Li—Mn composite oxide. X-raydiffraction analysis apparatus (XRD) revealed that in addition to Li—Mncomposite oxide, a trace amount of dimanganese trioxide was alsocontained in the synthesized product. The average particle size of theproduct measured by means of a laser particle size distributionmeasuring apparatus was 10 μm, and the specific surface area thereof was7.7 m²/g.

The obtained Li—Mn composite oxide having spinal structure was dispersedin ethanol solvent and pulverized with a wet ball mill such that theaverage particle size became 0.5 μm. Measurement revealed that particleshaving a particle size of 3 μm or more were not contained, and that thespecific surface area of the particles was 27.8 m²/g. The powder wasmixed with bismuth oxide having an average particle size of 2 μm so asto attain a Bi/Mn element ratio of 0.0026. The resultant mixture wasgranulated with Spartanryuzer RMO-6H (Fuji Paudal).

An aqueous solution of polyvinyl alcohol (1.5 parts by weight), servingas a granulation aid, was added to the mixed powder (100 parts byweight) of the Li—Mn composite oxide and bismuth oxide, and was thenfollowed by granulation for 16 minutes. The obtained granulatedsubstance was lightly crushed and then pulverized in a mixer, to therebyobtain powder having an average particle size of 15 μm as measured by apneumatic classifier. The tapping density of the size-adjusted granuleswas 1.65 g/ml.

The resultant granules were left to stand under atmospheric condition,in air, at 500° C. for two hours, to thereby remove the binder from thegranules (i.e., to decompose polyvinyl alcohol in the granules).Thereafter, the resultant granules were sintered at 750° C. in air to750° C. at a rate of 200° C./hour, and then maintained at 750° C. for 20hours, to hereby produce a cathode electroactive material. Inductivelycoupled plasma emission spectroscopy (ICP-AES) confirmed that elementalBi derived from the bismuth oxide was present in the electroactivematerial in an amount corresponding to the amount of bismuth oxideemployed.

The average porosity of the obtained cathode electroactive material wasfound to be 11.2%. The tapping density and crystallite size of thematerial were 1.96 g/ml and 880 Å, respectively. The lattice constantmeasured with respect to the cathode electroactive material was 8.233 Å.

Using the thus-obtained cathode electroactive material, a coin-shapedcell was fabricated as follows. The cathode electroactive material,carbon black serving as a conductor, and polyvinylidenefluoride inN-methyl-2-pyrrolidone were kneaded in proportions by weight of80:10:10. The resultant substance was applied to aluminum foil and thenpressed, to thereby obtain a cathode electrode. Lithium foil having apredetermined thickness was used as an anode electrode. Propylenecarbonate and dimethyl carbonate were mixed at a volume ratio of 1:2.LiPF₆ was dissolved in the obtained mixed liquid at a concentration of 1mol/liter. The resultant solution was used as an electrolyte. Using thethus-obtained cathode electrode, anode electrode, and electrolyte aswell as a polypropylene separator and a glass filter, a 2016-typecoil-shaped cell was fabricated.

The fabricated cell was tested at 60° C. over subjection to 100charge-discharge cycles, each performed at a charge-discharge rate of 1Cand within a voltage range of 3.0 V to 4.2 V. Table 1 shows the initialdischarge capacity and the capacity retention percentage (%) as measuredafter the 100-cycles test with the other results of measurement.

EXAMPLE 2

The procedure of Example 1 was repeated, except that electrolyticallyproduced manganese dioxide serving as a manganese source, to therebysynthesize an Li—Mn composite oxide. As is similar to Example 1,porosity, tapping density, crystallite size, and lattice constant of thesecondary particles, and electrode performance were evaluated. Theresults are shown in Table 1.

EXAMPLE 3

Manganese carbonate, lithium carbonate, and aluminum hydroxide weremixed together at proportions by element of 1.02:1.967:0.013 (Li/Mn/Al)using a ball mill. The resultant mixture was heated from roomtemperature to 650° C. at a rate of 200° C./hour in air. The temperaturewas maintained at 650° C. for four hours, to thereby synthesize an Li—Mncomposite oxide. XRD revealed that in addition to Li—Mn composite oxide,a trace amount of dimanganese trioxide was also contained in thesynthesized product. The average particle size of the product measuredby means of a laser particle size distribution measuring apparatus was10 μm.

The produced Li—Mn composite oxide was crushed to particles having anaverage particle size of 0.5 μm. Boron oxide was added to the particlesso as to adjust the element ratio (B/Mn) to 0.0208, and the mixture wasgranulated. Subsequently, the procedure of Example 1 was repeated,except that binder-free granulates were burned at 750° C. for 0.5 hour.The results of evaluation are shown in Table 1.

EXAMPLE 4

The procedure of Example 3 was repeated, except that the element ratio(B/Mn) was adjusted to 0.009 and binder-free granules were burned at760° C. for 0.5 hour. The results of evaluation are shown in Table 1.

EXAMPLE 5

The procedure of Example 3 was repeated, except that the element ratio(B/Mn) was adjusted to 0.006 and binder-free granules were burned at770° C. for 0.5 hour. The results of valuation are shown in Table 1.

EXAMPLE 6

The procedure of Example 1 was repeated, except that binder-freegranules were burned at 760° C. for 20 hours. The results of evaluationare shown in Table 1.

EXAMPLE 7

The procedure of Example 1 was repeated, except that tungsten trioxidewas used instead of bismuth oxide; tungsten trioxide was added in anelement ratio (W/Mn) of 0.0208; and binder-free granules were burned at750° C. for 20 hours. The results of evaluation are shown in Table 1.

EXAMPLE 8

The Li—Mn composite oxide which had been synthesized in Example 1 wasfurther heated from room temperature to 750° C. in air at a heating rateof 200° C./hour, and the thus-heated oxide was maintained at 750° C. for20 hours, to thereby crystallize. The procedure of Example 1 wasrepeated, except that crystallized Li—Mn composite oxide was used, boronoxide was used instead of bismuth oxide; boron oxide was added in anelement ratio (B/Mn) of 0.0208; and binder-free granules were burned at750° C. for 0.5 hour. The results of evaluation are shown in Table 1.

EXAMPLE 9

The procedure of Example 3 was repeated, except that Li—Mn compositeoxide particles having an average particle size of 3.5 μm and a specificsurface area of 10 m²/g were employed as ungranulated particles. Theresults of evaluation are shown in Table 1.

EXAMPLE 10

The procedure of Example 3 was repeated, except that manganesecarbonate, lithium carbonate, and aluminum hydroxide were mixed togetherat proportions by element of 1.03:1.967:0.013 (Li/Mn/Al) using a ballmill, to thereby synthesize an Li—Mn composite oxide. The results ofevaluation are shown in Table 1.

TABLE 1 60° C. Cell Cathode electroactive material performance Mol ratioBurning Specific Capacity of added conditions Tapping surfaceCrystallite Lattice Initial retention after sintering after Porositydensity area size constant Aspect capacity 100 cycles No agentbinder-free % g/ml m²/g Å Å Roundness ratio mAh/g % Ex. 1 Bi/Mn 750° C.× 11.2 1.96 1.8 880 8.233 0.76 1.31 129 84 0.0026  20 hr Ex. 2 Bi/Mn750° C. × 12.0 1.93 1.8 890 8.234 0.75 1.33 118 78 0.0026  20 hr Ex. 3Bi/Mn 750° C. × 6.5 2.16 1.2 780 8.232 0.78 1.28 127 85 0.0208 0.5 hrEx. 4 Bi/Mn 760° C. × 2.3 2.33 1.0 910 8.231 0.78 1.29 125 83 0.0090 0.5hr Ex. 5 Bi/Mn 770° C. × 1.8 2.35 0.9 930 8.230 0.77 1.28 126 81 0.00600.5 hr Ex. 6 Bi/Mn 760° C. × 9.1 2.05 1.2 910 8.231 0.78 1.29 115 870.0026  20 hr Ex. 7 Bi/Mn 750° C. × 6.1 2.18 1.1 800 8.239 0.78 1.28 12476 0.0208  20 hr Ex. 8 Bi/Mn 750° C. × 9.8 2.02 1.5 820 8.240 0.76 1.30128 80 0.0208 0.5 hr Ex. 9 Bi/Mn 750° C. × 8.5 2.07 1.3 750 8.233 0.791.31 126 85 0.0208 0.5 hr Ex. 10 Bi/Mn 750° C. × 6.3 2.15 1.2 750 8.2280.76 1.29 116 89 0.0208 0.5 hr

EXAMPLE 11

The procedure of Example 1 was repeated, except that binder-freegranules were burned at 830° C. for 20 hours. The results of evaluationare shown in Table 2.

EXAMPLE 12

The procedure of Example 3 was repeated, except that manganesecarbonate, lithium carbonate, and aluminum hydroxide were mixed togetherat proportions by element of 0.99:1.967:0.013 (Li/Mn/Al) using a ballmill, to thereby synthesize an Li—Mn composite oxide. The results ofevaluation are shown in Table 2.

EXAMPLE 13

The procedure of Example 3 was repeated, except that the averageparticle size of granulated particles was adjusted to 65 μm. The resultsof evaluation are shown in Table 2.

EXAMPLE 14

The procedure of Example 1 was repeated, except that the element ratio(Bi/Mn) was adjusted to 0.0020. The results of evaluation are shown inTable 2. The obtained cathode electroactive material, which wasgranulated, burned, and size-adjusted, was observed by a scanningelectron microscope (SEK)(×15,000). As shown in FIG. 1, the particleshave been found to be spherical. The particle size distribution of theseparticles is shown in FIG. 2.

COMPARATIVE EXAMPLE 1

The procedure of Example 1 was repeated, except that non-granulatedparticles of the Li—Mn composite oxide before undergoing granulation hadan average particle size of 6.0 μm. The results of evaluation are shownin Table 2.

COMPARATIVE EXAMPLE 2

Lithium carbonate and electrolytically synthesized manganese dioxidehaving an average particle size of 20 μM were mixed together at anelement ratio (Li/Mn) of 0.51 using a ball mill, and the mixture washeated to 760° C. at a heating rate of 100° C./hour and the heatedmixture was maintained at 760° C. for 24 hours, to thereby synthesize acathode electroactive material. The thus-obtained cathode electroactivematerial was evaluated in a manner similar to that employed inExample 1. The results are shown in Table 2.

COMPARATIVE EXAMPLE 3

The procedure of Example 1 was repeated, except that granulation wasperformed without adding a sintering agent. The results of evaluationare shown in Table 2.

COMPARATIVE EXAMPLE 4

The procedure of Example 3 was repeated, except that granules wereburned at 750° C. for 20 hours. The results of evaluation are shown inTable 2.

TABLE 2 60° C. Cell Mol ratio Cathode electroactive material performanceof Burning Specific Capacity sintering conditions Tapping surfaceCrystallite Lattice Initial retention after agent after Porosity densityarea size constant Aspect capacity 100 cycles No added binder-free %g/ml m²/g Å Å Roundness ratio mAh/g % Ex. 11 Bi/Mn 830° C. × 2.0 2.340.8 960 8.235 0.74 1.35 127 71 0.0026 20 hr Ex. 12 Bi/Mn 750° C. × 6.42.14 1.4 770 8.243 0.73 1.34 131 74 0.0208 0.5 hr  Ex. 13 Bi/Mn 750° C.× 6.6 2.48 1.2 790 8.233 0.84 1.25 128 85 0.0208 0.5 hr  Ex. 14 Bi/Mn750° C. × 15.0 1.83 2.0 850 8.235 0.74 1.32 131 84 0.0200 20 hr Comp.Bi/Mn 750° C. × 16.6 1.74 2.5 930 8.233 0.72 1.33 120 77 Ex. 1 0.0026 20hr Comp. — 760° C. × 16.0 1.71 6.8 600 8.239 0.66 1.45 110 73 Ex. 2 24hr Comp. — 750° C. × 19.6 1.62 4.8 580 8.232 0.71 1.32 125 83 Ex. 3 20hr Comp. Bi/Mn 750° C. × 5.4 2.20 0.3 >1000 8.234 0.69 1.39 122 54 Ex. 40.0208 20 hr

EXAMPLE 15

Manganese carbonate, lithium carbonate, and aluminum hydroxide weremixed together at proportions by element of 1.02:1.967:0.013 (Li/Mn/Al)using a ball mill. The resultant mixture was heated from roomtemperature to 650° C. at a rate of 200° C./hour in air. The temperaturewas maintained at 650° C. for four hours, to thereby synthesize an Li—Mncomposite oxide. XRD revealed that in addition to Li—Mn composite oxide,a trace amount of dimanganese trioxide was also contained in thesynthesized product. The average particle size of the product measuredby means of a laser particle size distribution measuring apparatus was10 μm.

Boron oxide was added to the obtained Li—Mn composite oxide so as toattain an element ratio (B/Mn) of 0.0208. The resultant mixture wasdispersed in ethanol solvent and pulverized with a wet ball mill suchthat the average particle size became 0.3 μm. The resultant mixture wasgranulated with Spartanryuzer RMO-6H (Fuji Paudal).

An aqueous solution of polyvinyl alcohol (1.5 parts by mass), serving asa granulation aid, was added to the mixed powder (100 parts by mass) ofthe Li—Mn composite oxide and boron oxide, and was then followed bygranulation for 16 minutes. The obtained granulated substance waslightly crushed and then pulverized in a mixer, to thereby obtain powderhaving an average particle size of 15 μm as measured by a pneumaticclassifier. The tapping density of the size-adjusted granules was 1.60g/ml.

The resultant granules were left to stand under atmospheric condition at500° C. for two hours, to thereby remove binders of the granules (i.e.,to decompose polyvinyl alcohol in the granules). Thermo-mechanicalanalysis of the binder-free granulates revealed thatsintering-shrinkage-initiating temperature of the granulates was 660° C.

Subsequently, binder-free granulates were sintered by use of a rotarykiln under the following conditions.

The temperature of the uniform-heat zone of the rotary kiln was adjustedto 780° C. Feeding rate of granules, and rotation speed and inclinationof the rotary kiln were tuned such that the binder-free granules passthrough the uniform-heat zone for three minutes. Time required fortransferring the binder-free granules from the inlet to the uniform-heatzone and that required for transferring the granules from theuniform-heat zone to the outlet of the kiln were 6.3 minutes,respectively.

The average porosity of the obtained cathode electroactive material wasfound to be 2.1%. The longest particle size of each of 500 primaryparticles was on an SEM image, and the average particle size was foundto be 0.40 μm.

Using the thus-obtained cathode electroactive material, a coin-shapedcell was fabricated as in the same way as in Example 1.

The fabricated cell was tested at 60° C. over 100 charge-dischargecycles, each performed at a charge-discharge rate of 1C and within avoltage range of 3.0 V to 4.2 V.

Table 3 shows the initial discharge capacity and the capacity retentionpercentage (%) as measured after the 100-cycle test.

EXAMPLE 16

The temperature of the uniform-heat zone of the rotary kiln was adjustedto 780° C. The procedure of Example 15 was repeated, except that feedingrate of granules and rotation speed and inclination of the rotary kilnwere tuned such that the debindered granules pass through theuniform-heat zone for nine minutes. The results of evaluation are shownin Table 3.

EXAMPLE 17

Manganese carbonate, lithium carbonate, and vapor-phase-synthesizedalumina were mixed together at proportions by element of1.02:1.967:0.013 (Li/Mn/Al) using a ball mill. The resultant mixture washeated from room temperature to 650° C. at a rate of 200° C./hour inair. The temperature was maintained at 650° C. for four hours, tothereby synthesize an Li—Mn composite oxide. XRD revealed that inaddition to Li—Mn composite oxide, a trace amount of dimanganesetrioxide was also contained in the synthesized product. The averageparticle size of the product measured by means of a laser particle sizedistribution measuring apparatus was 10 μm.

Boron oxide was added to the obtained Li—Mn composite oxide so as toattain an element ratio (B/Mn) of 0.0104. The resultant mixture wasdispersed in ion-exchange water, and pulverized with a medium-stirringmicro-pulverizer such that the average particle size became 0.18 μm.Granulation aid (Isobam 104 Kuraray Co., Ltd.) was added to theresultant slurry in an amount, of 1.5 masse based on the Li—Mn compositeoxide, and dry-granulation was carried out by use of a disk-rotatingspray-drier. The granulated substance was found to be sphericalparticles having an average particle size of 18.3 μm and a tappingdensity of 1.54 g/ml.

The thus-prepared granulates were allowed to stand under atmosphericcondition at 500° C. for two hours for removal of binders. Thebinder-free granules were sintered by use of a rotary kiln underconditions similar to those employed in Example 15.

The obtained cathode electroactive material was found to have an averageporosity of 1.7%, an average particle size of 0.27 μm, a tapping densityof 2.40 g/ml, and a specific surface area (BET) of 0.8 m²/g. Acoin-shaped cell was fabricated from the cathode electroactive materialin a manner similar to that employed in Example 15. The cell performanceis shown in Table 3.

EXAMPLE 18

The procedure of Example 15 was repeated, except that the temperature ofthe uniform-heat zone of a rotary kiln was adjusted to 850° C. Theresults of evaluation are shown in Table 3.

EXAMPLE 19

The procedure of Example 17 was repeated, except that the temperature ofthe uniform-heat zone of a rotary kiln was adjusted to 850° C. Theresults of evaluation are shown in Table 3.

COMPARATIVE EXAMPLE 5

The procedure of Example 15 was repeated, except that binder-freegranulates were heated from 650° C. to 750° C. at a rate of 10°C./minute, maintained at 750° C. for 0.5 hour, sintered, and cooled to650° C. at 10° C./minute. The obtained cathode electroactive materialwas evaluated in a manner similar to that employed in Example 15. Theresults of evaluation are shown in Table 3.

COMPARATIVE EXAMPLE 6

The procedure of Comparative Example 5 was repeated, except thatsintering was carried out at 750° C. for 20 hours. The results ofevaluation are shown in Table 3.

COMPARATIVE EXAMPLE 7

The temperature of the uniform-heat zone of the rotary kiln was adjustedto 780° C. The procedure of Example 15 was repeated, except that feedingrate of granules, and rotation speed and inclination of the rotary kilnwere tuned such that the binder-free granules pass through theuniform-heat zone for 0.5 minute, and time required for transferring thebinder-free granules from the inlet to the uniform-heat zone and thatrequired for transferring the granules from the uniform-heat zone to theoutlet of the kiln were 1.5 minutes, respectively. The results ofevaluation are shown in Table 3.

TABLE 3 60° C. Cell Cathode electroactive material performance Av.primary Specific Capacity Tapping particle surface Lattice Initialretention after Sintering Porosity density size area constant Aspectcapacity 100 cycles No conditions % g/ml μm m²/g Å Roundness ratio mAh/g% Ex. 15 780° C. × 3 min 2.1 2.35 0.40 0.8 8.233 0.77 1.31 127 90 120°C./min Rotary kiln Ex. 16 780° C. × 9 min 1.6 2.44 0.50 0.4 8.236 0.751.29 128 87 120° C./min Rotary kiln Ex. 17 780° C. × 3 min 1.7 2.40 0.280.8 8.235 0.99 1.02 127 91 120° C./min Rotary kiln Ex. 18 850° C. × 9min 1.4 2.51 2.66 0.3 8.237 0.73 1.29 128 86 120° C./min Rotary kiln Ex.19 850° C. × 9 min 1.3 2.52 2.41 0.2 8.237 0.75 1.29 128 87 120° C./minRotary kiln Comp. 750° C. × 0.5 hr 6.5 2.14 0.55 1.2 8.232 0.78 1.28 12785 Ex. 5  10° C./min Box furnace Comp. 750° C. × 20 hr 2.3 2.33 0.84 0.38.235 0.78 1.29 125 70 Ex. 6  10° C./min Box furnace Comp. 780° C. × 0.5min 13.1 1.92 0.19 2.1 8.233 0.77 1.28 118 81 Ex. 7 120° C./min RotarykilnMeasurement and Analysis of Shape of Granulated Particles

Through analysis of the secondary particles produced in Examples 1 to 19and Comparative Examples 1 to 7 shown in Tables 1 to 3, roundness(roundness=4π[area/(circumference)²]) and aspect ratio (aspectratio=absolute maximum length of needle/diagonal width) of secondaryparticles were obtained. The cathode electroactive materials accordingto the present invention were found to have a roundness of 0.7 or more,and an aspect ratio of 1.35 or less.

INDUSTRIAL APPLICABILITY

The cathode electroactive material of the present invention isdefinitely different from conventional electroactive material comprisingsecondary particles formed on the basis of cohesive force, since thecathode electroactive material of the present invention is producedthrough granulation and sintering. The material is formed of particleswhich are dense and spherical and exhibit excellent packingcharacteristics to an electrode, as compared with cathode electroactivematerial obtained through a conventional process for producing the same.In addition, the cathode electroactive material serves as a materialwhich enhances initial discharge capacity and capacity retentionpercentage of secondary cells even at high temperature.

The process of the present invention for producing a cathodeelectroactive material includes adding a sintering agent forming a meltat high temperature to the Li—Mn composite oxide, to thereby densifysecondary particles. The process of the present invention is alsoadvantageous as compared with conventional processes in that excellentcell performance can be attained even when crystallites have a sizewhich is detrimental to initial capacity and cycling characteristics.During densification of secondary particles, there is a problem thatprimary particles are grown to a particle size more than 0.5 μm, therebylowering initial capacity and cycling characteristics. The presentinvention can solve the problem by adding a sintering agent forming amelt at high temperature to the Li—Mn composite oxide, and provides acathode electroactive material having high packing characteristics andexcellent cell performance.

The lithium ion secondary cell of the present invention employs acathode electroactive material having an excellent packing property,accordingly, exhibits high initial capacity and capacity retentionpercentage at high temperature.

1. A process for producing a cathode electroactive material for use inlithium ion secondary cells which comprises adding, to a pulverizedLi—Mn composite oxide with a spinel structure, at least one componentselected from the group consisting of an oxide which is molten at 550°C.-900° C., an oxide which forms a solid solution with lithium ormanganese having a melting point of 550° C.-900° C., an oxide whichmelts to react with lithium or manganese to form a compound having amelting point of 550° C.-900° C., an element which forms one of theseoxides, and a compound which converts into one of these oxides; tothereby form granules and, wherein the granulation process is carriedout through spray granulation, agitation granulation, compressivegranulation, or fluidization granulation.
 2. A process for producing acathode electroactive material for use in lithium ion secondary cells asclaimed in claim 1, which process comprises sintering the granules inaddition to forming granules.
 3. A process for producing a cathodeelectroactive material for use in lithium ion secondary cells as claimedin claim 1, which process comprises, in addition to forming granules,sintering the granules by elevating the temperature of the granules froma sintering-shrinkage-initiating temperature to a temperature higherthan the sintering-shrinkage-initiating temperature by at least 100° C.at a rate of at least 100° C./minute; successively maintaining theelevated temperature for one minute-10 minutes; and lowering thetemperature to the sintering-shrinkage-initiating temperature at a rateof at least 100° C./minute.
 4. A process for producing a cathodeelectroactive material for use in lithium ion secondary cells as claimedin claim 3, wherein the sintering is carried out by use of a rotarykiln.
 5. A process for producing a cathode electroactive material foruse in lithium ion secondary cells as claimed in claim 2, wherein theoxide, the element or the compound comprises at least one selected fromthe group consisting of Bi, B, W, Mo, and Pb; a compound comprising B₂O₃and LiF; or a compound comprising MnF₂ and LiF.
 6. A process forproducing a cathode electroactive material for use in lithium ionsecondary cells as claimed in claim 1, wherein pulverized Li—Mncomposite oxide with the spinel structure has an average particle sizeof 5 μm or less.
 7. A process for producing a cathode electroactivematerial for use in lithium ion secondary cells as claimed in claim 1,wherein pulverized Li—Mn composite oxide with the spinel structure hasan average particle size of 3 μm or less.
 8. A process for producing acathode electroactive material for use in lithium ion secondary cells asclaimed in claim 1, wherein at least one organic compound selected fromthe group consisting of acrylic resin, an isobutylene-maleic anhydridecopolymer, poly(vinyl alcohol), poly(ethylene glycol),polyvinylpyrrolidene, hydroxypropyl cellulose, methyl cellulose,cornstarch, gelatin, and lignin is employed as a granulation aid duringgranulation process.
 9. A process for producing a cathode electroactivematerial for use in lithium ion secondary cells as claimed in claim 8,which process comprises removing the granulation aid in air or in anoxygen-containing environment at 300° C. to 550° C.
 10. A cathodeelectroactive material for use in lithium ion secondary cells which isproduced through a process as claimed in claim 1.